[LVAS] Turned Down Edge

[Fred Rayworth]

Roger,

 

Thanks for the kind words. 

 

I have no idea what the FOV of my finder is. It is a Meade 50mm finder given to me by Dave Blanchette. It is their standard finder for the SCT line. All I know is that it works great when I'm not using my laser. It used to work even greater BEFORE I got my laser.

 

The other night, I used it for a full hour before it got dark enough to see the laser.

 

Fred


 


From: drivester at hotmail.com
To: lvas at lvlug.org
Date: Thu, 18 Feb 2010 02:36:20 +0000
Subject: Re: [LVAS] Turned Down Edge



Fred,
 
Thank you for sharing this information. Not being a mirror maker I was not sure exactly what caused the problem. Your description is almost verbatim of some information that I have read in the past. If there is a turned down edge (the article said) you are not really losing anything by masking off 1/8 to 1/4 inch, as it was only scattering light.
 
Maybe you could share some more mirror making essays. I have always wanted to make a mirror. However, at this point I am more interested in learning more about it, and the problems that are encountered and how they affect the mirror, etc. 
 
By the way, what is the FOV in degree's (°) is your finder on your 16-inch? 
 
Good job in explaining a turned down edge...Roger    



 


From: rayworth1969 at hotmail.com
To: lvas at lvlug.org
Date: Wed, 17 Feb 2010 20:16:42 -0600
Subject: [LVAS] Turned Down Edge



All,
 
I was asked a question about turned down edge, and decided to answer it here.
 
Some of you may have heard that many if not most commercial mirrors have at least a slightly turned down edge. What does this mean? It means that not all of the light cone is going to come together at one spot. That wonky edge is going to throw some of the light away from the focal point, fuzzing the overall image just a bit. That affects contrast and focus. A Ronchi grating is a great way to see turned edge. One way or the other, a solution given many times is to mask the edge of the mirror in by 1/4" to 1/2". It can be done with flat black paint, press-on sticky paper, whatever.
 
What does this do? It decreases the size of your mirror by 1/2" to 1". But it also eliminates all that scattered light that the bad edge is throwing everywhere. There is also another bonus. It effectively increases the focal ratio. The longer the focal ratio, the sharper the images become and the less coma you have to deal with.
 
For instance, your mirror is a 10" f/4 and you mask 1/2" of the edge, effectively making it a 9". Your 40" focal length will give the 9" mirror a focal ratio of f/4.44. That slight increase in focal ratio takes away a bit of coma and thus slightly flattens and sharpens the image.
 
Keep in mind that this will not work if your mirror is astigmatic, has severe spherical abberation, is pinched, etc. However, if a turned edge is your only problem, this could turn your mediocre scope into a slightly smaller excellent scope.
 
Another feature of increasing the focal ratio is that if you DO have spherical abberation, that little stretch will make the abberation less severe. The longer the focal ratio, the more tolerant the center of the mirror is to error, up to a point. 
 
The larger the mirror, the more you can afford to cut out without making a significant difference in light gathering power. However, if you have a severely turned down edge, you could lose 2" to 3" of aperture just to get rid of it and then it is time to return the mirror and request a new one.
 
In the example above, you will likely see little difference between a 10" and 9" mirror except that it may be sharper. If you have a 16" mirror and have to mask it down to 14", them are fightin' words!
 
Fred
 



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